ce in bumpy donut-shaped experimental fusion facilities called stellarators. This week in Physical Review Letters, these authors describe an advanced application of the method that could help physicists overcome a major barrier to the production of fusion energy in such devices, and could also apply to their more widely used symmetrical donut-shaped cousins called tokamaks. This work was supported by the DOE Office of Science.

Turbulence allows the hot, charged plasma gas that fuels fusion reactions to escape from the magnetic fields that confine the gas in stellarators and tokamaks. This turbulent transport occurs at comparable levels in both devices, and has long been recognized as a challenge for both in producing fusion power economically.

“Confinement bears directly on the cost of fusion energy,” said physicist Harry Mynick, a PPPL coauthor of the paper, “and we’re finding how to reshape the plasma to enhance confinement.”

The new method uses two types of advanced computer codes that have only recently become available. The authors modified these codes to address turbulent transport, evolving the starting design of a fusion device into one with reduced levels of turbulence. The current paper applies the new method to the Wendelstein 7-X stellarator, soon to be the world’s largest when construction is completed in Greifswald, Germany.

Results of the new method, which has also been successfully applied to the design of smaller stellarators and tokamaks, suggest how reshaping the plasma in a fusion device could produce much better confinement. Equivalently, improved plasma shaping could produce comparable confinement with reduced magnetic field strength or reduced facility size, with corresponding reductions in the cost of construction and operation.

The simulations further suggest that a troublesome characteristic called “stiffness” could occur in reactor-sized stellarators. Stiffness, the tendency for heat to rapidly escape as the plasma temperature gradient rises above a threshold, has been observed in tokamaks but less so in stellarators. The possibility that stiffness might be present in reactor-sized stellarators, wrote the authors, could stimulate efforts “toward further optimizing stellarator magnetic fields for reduced turbulence.”

PPPL, on Princeton University’s Forrestal Campus in Plainsboro, New Jersey, is devoted to creating new knowledge about the physics of plasmas — ultra-hot, charged gases — and to developing practical solutions for the creation of fusion energy. Fusion takes place when atomic nuclei fuse and release a burst of energy. This compares with the fission reactions in today’s nuclear power plants, which operate by splitting atoms apart.

Results of PPPL research have ranged from a portable nuclear materials detector for anti-terrorist use to universally employed computer codes for analyzing and predicting the outcome of fusion experiments. The laboratory is managed by the University for the U.S. Department of Energy’s Office of Science, which is the single largest supporter of basic research in the physical sciences in the United States, and is working to address some of the most pressing challenges of our time.

Magnetic reconnection in the Earth and sun’s atmospheres can trigger geomagnetic storms that disrupt cell phone service, damage satellites and blackout power grids. Understanding how reconnection transforms magnetic energy into explosive particle energy has been a major unsolved problem in plasma astrophysics.

Scientists at the Princeton Plasma Physics Laboratory (PPPL) and Princeton University have taken a key step toward a solution, as described in a paper published this week in the journal NatureCommunications. In research conducted on the Magnetic Reconnection Experiment (MRX) at PPPL, the scientists not only identified how the mysterious transformation takes place, but measured experimentally the amount of magnetic energy that turns into particle energy. The work is supported by the U. S. Department of Energy as well as the NSF-funded Center for Magnetic Self-Organization.

Fast-camera image of plasma during magnetic reconnection with rendering of the field lines, shown in white, based on measurements made during the experiment. The converging horizontal lines represent the field lines prior to reconnection. The outgoing vertical lines represent the field lines after reconnection. Image courtesy of Jongsoo Yoo.

Magnetic field lines represent the direction, and indicate the shape, of magnetic fields. In magnetic reconnection, the magnetic field lines in plasma snap apart and violently reconnect. The MRX, built in 1995, allows researchers to study the process in a controlled laboratory environment.

The new research shows that reconnection converts about 50 percent of the magnetic energy, with one-third of the conversion heating the electrons and two-thirds accelerating the ions — or atomic nuclei — in the plasma. In large bodies like the sun, such converted energy can equal the power of millions of tons of TNT.

“This is a major milestone for our research,” said Masaaki Yamada, a research physicist, the principal investigator for the MRX and first author of the Nature Communications paper. “We can now see the entire picture of how much of the energy goes to the electrons and how much to the ions in a proto-typical reconnection layer.”

The findings also suggested the process by which the energy conversion occurs. Reconnection first propels and energizes the electrons, according to the researchers, and this creates an electrically charged field that “becomes the primary energy source for the ions,” said Jongsoo Yoo, an associate research physicist at PPPL and co-author of the paper.

The other contributors to the paper were Hantao Ji, professor of astrophysical sciences at Princeton; Russell Kulsrud, professor of astrophysical sciences, emeritus, at Princeton; and doctoral candidates in astrophysical sciences Jonathan Jara-Almonte and Clayton Myers.

If confirmed by data from space explorations, the PPPL results could help resolve decades-long questions and create practical benefits. These could include a better understanding of geomagnetic storms that could lead to advanced warning of the disturbances and an improved ability to cope with them. Researchers could shut down sensitive instruments on communications satellites, for example, to protect the instruments from harm.

Next year NASA plans to launch a four-satellite mission to study reconnection in the magnetosphere — the magnetic field that surrounds the Earth. The PPPL team plans to collaborate with the venture, called the Magnetospheric Multiscale (MMS) Mission, by providing MRX data to it. The MMS probes could help to confirm the laboratory’s findings.

PPPL, on Princeton University’s Forrestal Campus in Plainsboro, New Jersey, is devoted to creating new knowledge about the physics of plasmas — ultra-hot, charged gases — and to developing practical solutions for the creation of fusion energy. Fusion takes place when atomic nuclei fuse and release a burst of energy. This compares with the fission reactions in today’s nuclear power plants, which operate by splitting atoms apart.

Results of PPPL research have ranged from a portable nuclear materials detector for anti-terrorist use to universally employed computer codes for analyzing and predicting the outcome of fusion experiments. The laboratory is managed by the University for the U.S. Department of Energy’s Office of Science, which is the single largest supporter of basic research in the physical sciences in the United States, and is working to address some of the most pressing challenges of our time.

Alexander Glaser and Robert Goldston with the British Test Object. Credit: Elle Starkman/PPPL Communications Office

By John Greenwald, Princeton Plasma Physics Laboratory Office of Communications

A proven system for verifying that apparent nuclear weapons slated to be dismantled contained true warheads could provide a key step toward the further reduction of nuclear arms. The system would achieve this verification while safeguarding classified information that could lead to nuclear proliferation.

Scientists at Princeton University and the U.S. Department of Energy’s (DOE) Princeton Plasma Physics Laboratory (PPPL) are developing the prototype for such a system, as reported this week in the journal Nature. Their novel approach, called a “zero-knowledge protocol,” would verify the presence of warheads without collecting any classified information at all.

“The goal is to prove with as high confidence as required that an object is a true nuclear warhead while learning nothing about the materials and design of the warhead itself,” said physicist Robert Goldston, coauthor of the paper, a fusion researcher and former director of PPPL, and a professor of astrophysical sciences at Princeton.

While numerous efforts have been made over the years to develop systems for verifying the actual content of warheads covered by disarmament treaties, no such methods are currently in use for treaty verification.

Traditional nuclear arms negotiations focus instead on the reduction of strategic — or long-range — delivery systems, such as bombers, submarines and ballistic missiles, without verifying their warheads. But this approach could prove insufficient when future talks turn to tactical and nondeployed nuclear weapons that are not on long-range systems. “What we really want to do is count warheads,” said physicist Alexander Glaser, first author of the paper and an assistant professor in Princeton’s Woodrow Wilson School of Public and International Affairs and the Department of Mechanical and Aerospace Engineering.

The system Glaser and Goldston are mapping out would compare a warhead to be inspected with a known true warhead to see if the weapons matched. This would be done by beaming high-energy neutrons into each warhead and recording how many neutrons passed through to detectors positioned on the other side. Neutrons that passed through would be added to those already “preloaded” into the detectors by the warheads’ owner — and if the total number of neutrons were the same for each warhead, the weapons would be found to match. But different totals would show that the putative warhead was really a spoof. Prior to the test, the inspector would decide which preloaded detector would go with which warhead.

No classified data would be measured in this process, and no electronic components that might be vulnerable to tampering and snooping would be used. “This approach really is very interesting and elegant,” said Steve Fetter, a professor in the School of Public Policy at the University of Maryland and a former White House official. “The main question is whether it can be implemented in practice.”

A project to test this approach is under construction at PPPL. The project calls for firing high-energy neutrons at a non-nuclear target, called a British Test Object, that will serve as a proxy for warheads. Researchers will compare results of the tests by noting how many neutrons pass through the target to bubble detectors that Yale University is designing for the project. The gel-filled detectors will add the neutrons that pass through to those already preloaded to produce a total for each test.

The project was launched with a seed grant from The Simons Foundation of Vancouver, Canada, that came to Princeton through Global Zero, a nonprofit organization. Support also was provided by the U.S. Department of State, the DOE (via PPPL pre-proposal development funding), and most recently, a total of $3.5 million over five years from the National Nuclear Security Administration.

Glaser hit upon the idea for a zero-knowledge proof over a lunch hosted by David Dobkin, a computer scientist, and until June 2014, dean of the Princeton faculty. “I told him I was really interested in nuclear warhead verification without learning anything about the warhead itself,” Glaser said. ‘“We call this a zero-knowledge proof in computer science,”’ Glaser said Dobkin replied. “That was the trigger,” Glaser recalled. “I went home and began reading about zero-knowledge proofs,” which are widely used in applications such as verifying online passwords.

Glaser’s reading led him to Boaz Barak, a senior researcher at Microsoft New England who had taught computer science at Princeton and is an expert in cryptology, the science of disguising secret information. “We started having discussions,” Glaser said of Barak, who helped develop statistical measures for the PPPL project and is the third coauthor of the paper in Nature.

Glaser also reached out to Goldston, with whom he had taught a class for three years in the Princeton Department of Astrophysical Sciences. “I told Rob that we need neutrons for this project,” Glaser recalled. “And he said, ‘That’s what we do — we have 14 MeV [or high-energy] neutrons at the Laboratory.’” Glaser, Goldston and Barak then worked together to refine the concept, developing ways to assure that even the statistical noise — or random variation — in the measurements conveyed no information.

If proven successful, dedicated inspection systems based on radiation measurements, such as the one proposed here, could help to advance disarmament talks beyond the New Strategic Arms Reduction Treaty (New START) between the United States and Russia, which runs from 2011 to 2021. The treaty calls for each country to reduce its arsenal of deployed strategic nuclear arms to 1,550 weapons, for a total of 3,100, by 2018.

Not included in the New START treaty are more than 4,000 nondeployed strategic and tactical weapons in each country’s arsenal. These very weapons, note the authors of the Nature paper, are apt to become part of future negotiations, “which will likely require verification of individual warheads, rather than whole delivery systems.” Deep cuts in the nuclear arsenals and the ultimate march to zero, say the authors, will require the ability to verifiably count individual warheads.

An electron microscope image shows two lasers placed just two microns apart from each other. (Image source: Turecki lab)

Inspired by anomalies that arise in certain mathematical equations, researchers have demonstrated a laser system that paradoxically turns off when more power is added rather than becoming continuously brighter.

The finding by a team of researchers at Vienna University of Technology and Princeton University, could lead to new ways to manipulate the interaction of electronics and light, an important tool in modern communications networks and high-speed information processing.

The researchers published their results June 13 in the journal Nature Communications.

Their system involves two tiny lasers, each one-tenth of a millimeter in diameter, or about the width of a human hair. The two are nearly touching, separated by a distance 50 times smaller than the lasers themselves. One is pumped with electric current until it starts to emit light, as is normal for lasers. Power is then added slowly to the other, but instead of it also turning on and emitting even more light, the whole system shuts off.

“This is not the normal interference that we know,” said Hakan Türeci, assistant professor of electrical engineering at Princeton, referring to the common phenomenon of light waves or sound waves from two sources cancelling each other. Instead, he said, the cancellation arises from the careful distribution of energy loss within an overall system that is being amplified.

Manipulating minute areas of gain and loss within individual lasers (shown as peaks and valleys in the image), researchers were able to create paradoxical interactions between two nearby lasers.(Image source: Turecki lab)

“Loss is something you normally are trying to avoid,” Türeci said. “In this case, we take advantage of it and it gives us a different dimension we can use – a new tool – in controlling optical systems.”

The research grows out of Türeci’s longstanding work on mathematical models that describe the behavior of lasers. In 2008, he established a mathematical framework for understanding the unique properties and complex interactions that are possible in extremely small lasers – devices with features measured in micrometers or nanometers. Different from conventional desk-top lasers, these devices fit on a computer chip.

That work opened the door to manipulating gain or loss (the amplification or loss of an energy input) within a laser system. In particular, it allowed researchers to judiciously control the spatial distribution of gain and loss within a single system, with one tiny sub-area amplifying light and an immediately adjacent area absorbing the generated light.

Türeci and his collaborators are now using similar ideas to pursue counterintuitive ideas for using distribution of gain and loss to make micro-lasers more efficient.

The researchers’ ideas for taking advantage of loss derive from their study of mathematical constructs called “non-Hermitian” matrices in which a normally symmetric table of values becomes asymmetric. Türeci said the work is related to certain ideas of quantum physics in which the fundamental symmetries of time and space in nature can break down even though the equations used to describe the system continue to maintain perfect symmetry.

Over the past several years, Türeci and his collaborators at Vienna worked to show how the mathematical anomalies at the heart of this work, called “exceptional points,” could be manifested in an actual system. In 2012 (Ref. 3), the team published a paper in the journal Physical Review Letters demonstrating computer simulations of a laser system that shuts off as energy is being added. In the current Nature Communications paper, the researchers created an experimental realization of their theory using a light source known as a quantum cascade laser.

The researchers report in the article that results could be of particular value in creating “lab-on-a-chip” devices – instruments that pack tiny optical devices onto a single computer chip. Understanding how multiple optical devices interact could provide ways to manipulate their performance electronically in previously unforeseen ways. Taking advantage of the way loss and gain are distributed within tightly coupled laser systems could lead to new types of highly accurate sensors, the researchers said.

“Our approach provides a whole new set of levers to create unforeseen and useful behaviors,” Türeci said.

The work at Vienna, including creation and demonstration of the actual device, was led by Stefan Rotter at Vienna along with Martin Brandstetter, Matthias Liertzer, C. Deutsch, P. Klang, J. Schöberl, G. Strasser and K. Unterrainer. Türeci participated in the development of the mathematical models underlying the phenomena. The work on the 2012 computer simulation of the system also included Li Ge, who was a post-doctoral researcher at Princeton at the time and is now an assistant professor at City University of New York.

The work was funded by the Vienna Science and Technology Fund and the Austrian Science Fund, as well as by the National Science Foundation through a major grant for the Mid-Infrared Technologies for Health and the Environment Center based at Princeton and by the Defense Advanced Research Projects Agency.

Princeton University researchers found that the “landscape” for quantum control (above) — a representation of quantum mechanics that allows the dynamics of atoms and molecules to be manipulated — can be unexpectedly simple, which could help scientists realize the next generation of technology by harnessing atoms and molecules to create small but incredibly powerful devices. Scientists achieve quantum control by finding the ideal radiation field (top of the graphic) that leads to the desired response from the system. Like a mountain hiker, a scientist can take a difficult, twisting path that requires frequent stops to evaluate the next step (right path). Or, they can opt for a straighter trail that cuts directly to the summit (left path). The researchers provide in their paper an algorithm that scientists can use to identify the starting point of the straight path to their desired quantum field. (Image courtesy of Arun Nanduri)

By Morgan Kelly, Office of Communications

Like any task, there is an easy and a hard way to control atoms and molecules as quantum systems, which are driven by tailored radiation fields. More efficient methods for manipulating quantum systems could help scientists realize the next generation of technology by harnessing atoms and molecules to create small but incredibly powerful devices such as molecular electronics or quantum computers.

Of course, controlling quantum systems is as painstaking as it sounds, and requires scientists to discover the ideal radiation field that leads to the desired response from the system. Scientists know that reaching that state of quantum nirvana can be a long and expensive slog, but Princeton University researchers have found that the process might be more straightforward than previously thought.

The researchers report in the journal Physical Review A that quantum-control “landscapes” — the path of a system’s response from the initial field to the final desired field — appears to be unexpectedly simple. Although still a mountain of a task, finding a good control radiation field turns out to be very much like climbing a mountain, and scientists need only choose the right path. Like a hiker, a scientist can take a difficult, twisting path that requires frequent stops to evaluate which step to take next. Or, as the Princeton researchers show, they can opt for a straighter trail that cuts directly to the summit.

The researchers observe in their paper that these fast tracks toward the desired control field actually exist, and are scattered all over the landscape. They provide an algorithm that scientists can use to identify the starting point of the straight path to their desired quantum field.

The existence of nearly straight paths to reach the best quantum control was surprising because the landscapes were assumed to be serpentine, explained first author Arun Nanduri, who received his bachelor’s degree in physics from Princeton in 2013 and is working in the laboratory of Herschel Rabitz, Princeton’s Charles Phelps Smyth ’16 *17 Professor of Chemistry.

“We found that not only can you always climb to the top, but you can climb along a simple path to the top,” Nanduri said. “If we could consistently identify where these paths are located, a scientist could efficiently climb the landscape. Looking around for the next good step along an unknown path takes great effort. However, starting along a straight path requires you to look around once, and you can keep walking forward with your eyes closed, as it were.”

Following a straighter path could be a far more efficient way of achieving control of atoms and molecules for a host of applications, including manipulating chemical reactions and operating quantum computers, Nanduri said. The source of much scientific excitement, quantum computers would use “qubits” that can be entangled to potentially give them enormous storage and computational capacities far beyond the capabilities of today’s digital computers.

If the Princeton research helps scientists quickly and easily find the control fields they need, it could also allow them to carry out improved measurements of quantum systems and design new ones, Nanduri said.

“We don’t know if our discovery will directly lead to futuristic quantum devices, but this finding should spur renewed research,” Nanduri said. “If straight paths to good quantum control solutions can be routinely found, it would be remarkable.”

By John Greenwald, Office of Communications, Princeton Plasma Physics Laboratory

A key issue for the development of fusion energy to generate electricity is the ability to confine the superhot, charged plasma gas that fuels fusion reactions in magnetic devices called tokamaks. This gas is subject to instabilities that cause it to leak from the magnetic fields and halt fusion reactions.

Now a recently developed imaging technique can help researchers improve their control of instabilities. The new technique, developed by physicists at the U.S. Department of Energy’s Princeton Plasma Physics Laboratory (PPPL), the University of California-Davis and General Atomics in San Diego, provides new insight into how the instabilities respond to externally applied magnetic fields.

This technique, called Electron Cyclotron Emission Imaging (ECEI) and successfully tested on the DIII-D tokamak at General Atomics, uses an array of detectors to produce a 2D profile of fluctuating electron temperatures within the plasma. Standard methods for diagnosing plasma temperature have long relied on a single line of sight, providing only a 1D profile. Results of the ECEI technique, recently reported in the journal Plasma Physics and Controlled Fusion, could enable researchers to better model the response of confined plasma to external magnetic perturbations that are applied to improve plasma stability and fusion performance.

As the universe expands, it is continually subjected to energy shifts, or “quantum fluctuations,” that send out little pulses of “sound” into the fabric of spacetime. In fact, the universe is thought to have sprung from just such an energy shift.

A recent paper in the journal Physical Review Letters reports a new mathematical tool that should allow one to use these sounds to help reveal the shape of the universe. The authors reconsider an old question in spectral geometry that asks, roughly, to what extent can the shape of a thing be known from the sound of its acoustic vibrations? The researchers approached this problem by breaking it down into small workable pieces, according to author Tejal Bhamre, a Princeton University graduate student in the Department of Physics.

To understand the authors’ method, consider a vase. If one taps a vase with a spoon, it will make a sound that is characteristic of its shape. Similarly, the technique Bhamre and her coauthors developed could, in principle, determine the shape of spacetime from the perpetual ringing caused by quantum fluctuations.

The researchers’ technique also provides a unique connection between the two pillars of modern physics — quantum theory and general relativity — by using vibrational wavelengths to define the geometric property that is spacetime.

Bhamre worked with coauthors David Aasen, a physics graduate student at Caltech, and Achim Kempf, a Waterloo University professor of physics of information.